30 or so years ago I saw a documentary of a mouse submerged in a tank of liquid, walking around on the bottom.........breathing the fluid as if it was normal. It's one of those things that made such an impression on me I've never forgotten it.
Then I saw the movie "The Abyss", and was moved to get online and research what was true, and what was "Hollywood".
Diving to extreme depths is dangerous for numerous reasons. One of the problems is our body's inability to rid itself of nitrogen that becomes dissolved as a fluid in the bloodstream when under pressure. If the diver returns to the surface too quickly, this liquified nitrogen returns to a gaseous state in places like knees and elbows. It causes extreme discomfort and sometimes crippling injury........"the bends".
To avoid this problem, divers follow time schedules laid out in decompression tables....charts that show the amount of time you must remain at a certain pressure level to allow the nitrogen to leave your body naturally. If you're not a diver, you would be surprised at the length of time involved here. It can literally be days in the case of some deep dives. Decompression can either be done underwater, which requires LOTS of air tanks, or in a decompression chamber if one is available.
Breathing a liquid instead of a gas could be beneficial in eliminating the nitrogen and therefore the need to decompress for such long periods. Highly oxygenated liquids, being incompressible, would also allow divers to go to more extreme depths.
But we're not quite there yet. The weight of liquids makes them difficult to move in and out of our lungs. Clearing the liquid from the lungs in order to return to breathing air is problematic.
But the video of the mouse running around submerged in that tank still fascinates me. It'll be amazing to see humans, rather than mice breathing while submerged!